Such laser-based hair cutting devices are, e.g., disclosed in the international patent application published as WO 2011/010246. Said international patent application describes such a device for shortening hairs comprising a laser source for generating a laser beam during a predetermined pulse time, an optical system for focusing the laser beam into a focus spot and a laser beam manipulator for positioning the focal spot in the target position. A dimension of the focal spot and a power of the generated laser beam are such that in the focal spot the laser beam has a power density which is above a characteristic threshold value for hair tissue, above which, for the predetermined pulse time, a laser-induced optical breakdown (LIOB) phenomenon occurs in the hair tissue. An optical blade with a tapered end guides the laser beam in a direction substantially parallel to the skin surface towards the hairs that are cut at a height just above the skin.
In general, laser-induced optical breakdown (LIOB) occurs in media, which are transparent or semi-transparent for the wavelength of a pulsed laser beam, when the power density of the laser beam in the focal spot exceeds a threshold value which is characteristic for the particular medium. Below the threshold value, the particular medium has relatively low linear absorption properties for the particular wavelength of the laser beam.
Above the threshold value, the medium has strongly non-linear absorption properties for the particular wavelength of the laser beam, which are the result of ionization of the medium and the formation of plasma. This LIOB phenomenon results in a number of mechanical effects, such as cavitation and the generation of shock waves, which damage the medium in positions surrounding the position of the LIOB phenomenon.
From experiments it appeared that the LIOB phenomenon can be used to break and shorten hairs growing from skin. Hair tissue is transparent or semi-transparent for wavelengths between approximately 500 nm and 2000 nm. For each value of the wavelength within this range, LIOB phenomena occur in the hair tissue at the location of the focal spot when the power density of the laser beam in the focal spot exceeds a threshold value which is characteristic for the hair tissue. Said threshold value is rather close to the threshold value which is characteristic for aqueous media and tissue and is dependent on the pulse time of the laser beam. In particular, the threshold value of the required power density (W/cm2) decreases when the pulse time increases. It appeared that in order to achieve mechanical effects as a result of the LIOB phenomenon which are sufficiently effective to cause significant damage, i.e. at least initial breakage of a hair, a pulse time in the order of, for example, 10 ns suffices. For this value of the pulse time, the threshold value of the power density of the laser beam in the focal spot is in the order of 2*1010 W/cm2. For the described pulse time and with a sufficiently small focal spot size obtained, for example, by means of a lens having a sufficiently large numerical aperture, this threshold value can be achieved with a total pulse energy of only a few tenths of a mJ.
Whilst it is possible, using the device of WO 2011/010546, to generate a laser-induced optical breakdown (LIOB) from an incident beam leaving the device through the exit surface of an optical blade and having sufficient energy to cut human hairs, the products of the LIOB (shock wave, plasma, high irradiance) can cause destructive damage to the exit surface. A damaged blade has a detrimental effect on the ability of the device to provide a tight focus at the desired position, which may reduce the efficacy of the shaving process and/or may increase the occurrence of adverse side effects, such as skin irritation.